There has been a strong demand for reduction of carbon dioxide in order to deal with atmospheric pollution and global warming. In the automobile industry, the reduction of emissions of carbon dioxide is highly expected in association with the spread of electric vehicles (EV) and hybrid electric vehicles (HEV). Thus, development of electric devices such as secondary batteries for driving motors as a key to practical application of such vehicles, is actively being carried out.
In particular, lithium ion secondary batteries are preferably used for electric vehicles because of high energy density and durability of repeated charge and discharge and therefore, the batteries tend to have larger capacities. As a result, the lithium ion secondary batteries are further required to ensure safety.
Lithium ion secondary batteries generally include positive electrodes each containing a positive electrode active material applied to both surfaces of a positive electrode current collector and negative electrodes each containing a negative electrode active material applied to both surfaces of a negative electrode current collector. Each positive electrode and each negative electrode are connected to each other via an electrolyte layer including a separator holding an electrolysis solution or electrolyte gel. Plural sets of the positive electrode, the negative electrode and the separator are housed in a battery casing.
For example, a polyolefin microporous film having a thickness approximately in the range from 20 μm to 30 μm is widely used as a separator. However, such a polyolefin microporous film has a risk of thermal contraction because of an increase in temperature inside the battery and a short circuit in association with the thermal contraction.
In order to suppress the thermal contraction, advances in the development of separators having heat resistant insulation layers are being made, in which heat resistant porous layers are stacked on surfaces of a microporous film. For example, Patent Literature 1 teaches that such a separator is used for a wound type lithium ion battery so as to suppress thermal contraction caused by an increase in temperature inside the battery.